For a display apparatus such as a flat panel display or a lighting fixture such as a back light, an organic light-emitting device emitting an electro-luminescence from a thin film made from an organic compound has been noted as a device by which a bright luminescence can be obtained at a low voltage.
Research and development for an increase in surface area employing a luminescent organic polymer soluble in an organic solvent or water are performed actively with respect to a styrene-based and a fluorene-based organic polymer as a material of a film in an organic light-emitting device. As a method for forming a film of such an organic polymer, a wet process such as a spin-coating method, a printing method, and an ink-jet method have been utilized. In particular, the ink-jet method is envisaged as a realistic method for creating pixels of a display screen in a full color display and a compact full-color prototype panel has already been disclosed.
On the other hand, research for great improvement of emission efficiency with respect to low-molecular compounds of which a film is formed by a vacuum deposition method has been performed actively and an organometallic compound with platinum or iridium, utilizing phosphorescence that is an emission from a triplet excited state of an organic compound, has been reported. An external emission quantum yield for an organic light-emitting device utilizing this phosphorescent compound exceeds 5% for a conventional device utilizing fluorescence and a high efficiency of 8% was obtained recently, and even a very high efficiency of 15% has been obtained by devising a structure of a device (Appl. Phys. Lett., 77, 904 (2000)).
There is a research report for a dope-type organic polymer light-emitting device in which this low-molecular phosphorescent compound is dispersed into an organic polymer, and for such a device that an iridium complex is doped into poly(N-vinylcarbazole) (PVK), the value of approximately 4% was obtained as an external emission quantum yield and a large improvement is appreciated (Jpn. J. Appl. Phys, 39, L28 (2000)). Also, for an organic polymer containing a ruthenium complex, an electrochemical luminescence was reported (J. Mater. Chem., 9, 2103 (1999)).
However, a conventional organic polymer light-emitting device, for which it is intended to realize an increase in surface area at low cost, has not been sufficient in respect to emission efficiency. The cause is that the theoretical external emission efficiency has an upper limit of 5% since fluorescence that is luminescence from a singlet excited state of a conventional organic polymer is utilized. Although an organic polymer light-emitting device has a major feature in that a film of an organic polymer layer can be formed from a solution in an organic solvent or water by a wet process, further improvement of emission efficiency is a problem for practice in the future.
Also, gaining great improvement of emission efficiency has been tried by dispersing a low-molecular phosphorescent compound into an organic polymer light-emitting device, but the low-molecular one dispersed into a host polymer is not stable and a display apparatus with a long service life and a reliability over a long period cannot be realized.
Therefore, taking the future realization of an organic light-emitting device into consideration, development of a novel organic polymeric luminescent material is desired, of which a film is formed from a solution in an organic solvent or water by a wet process, thereby an increase in surface area can be realized at low cost, which is stable, that is, has a long service life, and for which very highly efficient emission can be realized.